The invention relates to a device and a method for deNOxing exhaust gas from an internal combustion engine.
The reduction of the nitrogen oxide emissions from an internal combustion engine that operates with excess air, in particular, a diesel internal combustion engine, can be effected with the aid of selective catalytic reduction (SCR), to form atmospheric nitrogen (N2) and water vapor (H2O). The reducing agents used are either gaseous ammonia (NH3), ammonia in aqueous solution, or urea in aqueous solution. The urea serves as an ammonia carrier and is injected into the exhaust system with the aid of a metering system upstream of a hydrolysis catalytic converter, where it is converted into ammonia by hydrolysis, and the ammonia then reduces the nitrogen oxides in the actual SCR or deNOx catalytic converter.
The important components of such a metering system are a reducing-agent vessel, a pump, a pressure sensor, and a metering valve. The pump conveys the reducing agent stored in the reducing-agent vessel to the metering valve, by which the reducing agent is injected into the exhaust-gas stream upstream of the hydrolysis catalytic converter. The metering valve is actuated through signals from a control device such that a defined, currently required amount of reducing agent is supplied as a function of operating parameters of the internal combustion engine (German Patent DE 197 43 337 C1, corresponding to U.S. Pat. No. 6,082,102 to Wissler et al.).
An advantage of the ammonia-releasing substances that are present in aqueous solutions, such as, for example, urea, is that the storage, handling, delivery, and metering are, in technical terms, relatively simple to implement. A drawback of these aqueous solutions is that, in the event of heating above a defined temperature limit, which in turn is dependent, inter alia, on the concentration of the-dissolved substance, thermal decomposition of the solution starts to occur in the reducing-agent tank.
At high temperatures, for example, when the vehicle equipped with an exhaust-gas aftertreatment installation of this type is parked at locations with high insolation, or even while the vehicle is operating in hot regions, the reducing agent, which can be at least partially converted into ammonia, may be overheated. The decomposition vapor pressure, which increases as the temperature rises, for, for example, an aqueous urea solution, leads to the formation of ammonia and, therefore, to an increase in pressure in the reservoir.
In order, on one hand, to prevent the reservoir from being destroyed by an unacceptably high pressure and, on the other hand, to prevent slippage of ammonia, in particular, when the filler neck of the reservoir is opened, European Patent Application EP 0 577 853 B1 discloses, in an exhaust-gas aftertreatment installation for an internal combustion engine of the type described in the introduction, connecting a pressure-relief line, which feeds excess reducing agent to the deNOx catalytic converter, to the reservoir for the reducing agent. The pressure-relief line is connected to the inlet of the deNOx catalytic converter, i.e., to the side that faces the internal combustion engine. A pressure-control valve is incorporated in the pressure-relief line. As a result, the amount of excess ammonia that is to be received by the deNOx catalytic converter can be limited within the scope of the compressive strength of the reservoir.
In the prior art pressure relief method, although it is possible to avoid an unacceptably high build-up of pressure in the reservoir, the amount of reducing agent that is fed to the catalytic converter through the pressure-relief line can only be taken into account to an insufficient extent during the metering strategy.
It is accordingly an object of the invention to provide a device and method for deNOxing exhaust gas from an internal combustion engine that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that reliably prevents an unacceptably is high pressure in a reducing-agent reservoir of an exhaust-gas aftertreatment device of the type described in the introduction without impairing the metering accuracy.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for deNOxing exhaust gas from an internal combustion engine having an exhaust pipe conveying exhaust gas in an exhaust direction includes a reduction catalytic converter operating under an SCR principle, the converter, disposed in the exhaust pipe, a reducing-agent reservoir for holding a reducing agent, a metering device for introducing the reducing agent into exhaust gas flowing to the converter, a reducing-agent pump for delivering the reducing agent from the reservoir to the metering device, the pump fluidically connecting the reservoir to the metering device, the metering device fluidically connecting the pump to the exhaust pipe upstream of the converter with respect to the exhaust direction, a pressure-relief line for feeding excess reducing agent from the reservoir to the converter, the pressure-relief line fluidically connecting the reservoir to the converter, and a flow-measuring device for recording an amount of excess reducing agent passing through the pressure-relief line, the flow-measuring device disposed in the pressure-relief line.
As a result of the amount of gaseous reducing agent that escapes while the internal combustion engine is at a stand still, due to temperature influences being taken into account during the determination of the amount of reducing-agent solution that is to be metered when the internal combustion engine is operating, not only is the operational reliability increased, even in the range of critical ambient conditions, for example, in summertime operation, but also a high metering accuracy is achieved.
The targeted utilization of the gaseous reducing agent that is released by the heating, i.e., ammonia, when an aqueous urea solution is used as reducing agent, prevents slippage of reducing agent because, when a predetermined pressure level is reached in the reducing-agent reservoir, the gaseous reducing agent is passed into the reduction catalytic converter through a pressure-relief line. The amount of gaseous reducing agent that flows in is advantageously recorded by a flowmeter in the pressure-relief line and is taken into account during the calculation of the amount of reducing agent. For example, when the internal combustion engine is operating, liquid reducing agent is only injected again in a controlled manner into the exhaust pipe of the internal combustion engine when the gaseous reducing agent in the reduction catalytic converter has been consumed.
When the vehicle is parked, the values for pressure and opening time of a valve device disposed in the pressure-relief line can be stored by an intelligent sensor configuration and, after the internal combustion engine has been started these values are interrogated by a control unit that controls the metering of the reducing agent, are transmitted and the stored current reduction catalytic converter level can be corrected accordingly.
In accordance with another feature of the invention, the pressure-relief line has a cross-sectional opening for conveying the excess reducing agent, a pressure-control valve is disposed in the pressure-relief line, and the valve opens the cross-sectional opening when a predetermined pressure exists in the reservoir to permit the excess reducing agent to pass through the pressure-relief line.
In accordance with a further feature of the invention, the pressure-relief line has a cross-sectional opening for conveying the excess reducing agent, an electrically controllable valve) is disposed in the pressure-relief line, and the valve opens the cross-sectional opening when a predetermined pressure exists in the reservoir to permit the excess reducing agent to pass through the pressure-relief line.
In accordance with an added feature of the invention, the is flow-measuring device is a flowmeter, preferably, for ammonia.
In accordance with an additional feature of the invention, the pressure-relief line has an exit opening inside the converter.
In accordance with yet another feature of the invention, the reducing agent is a liquid.
With the objects of the invention in view, there is also provided a method for deNOxing exhaust gas from an internal combustion engine including the steps of determining, as a function of operating parameters of at least one of the internal combustion engine and a reduction catalytic converter operating under an SCR principle, an amount of reducing-agent solution to be metered and introducing the amount of the reducing agent into an exhaust pipe upstream of the converter while the internal combustion engine is operating, when the internal combustion engine is at a stand-still, feeding gaseous reducing agent formed as a result of temperature effects to the converter, and recording and taking into account an amount of the gaseous reducing agent during a determination of an amount of reducing-agent solution to be metered during operation of the internal combustion engine.
In accordance with yet a further feature of the invention, the gaseous reducing agent is supplied at a location inside the converter through a pressure-relief line connecting a reducing-agent reservoir and the converter.
In accordance with yet an added feature of the invention, the gaseous reducing agent is fed to the converter when a pressure in the reservoir exceeds a predetermined pressure level.
In accordance with yet an additional feature of the invention, the pressure-relief line is opened with a valve device disposed in the pressure-relief line when the predetermined pressure level is reached.
In accordance with again another feature of the invention, an amount of the gaseous reducing agent is determined with a flow-measuring device disposed in the pressure-relief line.
In accordance with again a further feature of the invention, an amount of the gaseous reducing agent is determined from a value for the pressure in the reservoir and a duration of an opening of the valve device.
In accordance with a concomitant feature of the invention, the reducing agent is aqueous urea solution and the flow-measuring device is a flowmeter for ammonia.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device and method for deNOxing exhaust gas from an internal combustion engine, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.